1. Field of the Invention
The present invention relates to a mercury removal system and mercury removal process for removing mercury contained in a gas discharged from combustion equipment.
2. Background Art
Exhaust gases released when coal or heavy oil is burnt contain ash dust, SOx (sulfur oxides) and NOx (nitrogen oxides) and they sometimes contain metallic mercury.
For treating this metallic mercury, a variety of processes or apparatuses have recently been devised, used in combination with a denitration apparatus for reducing NOx and a wet desulfurization apparatus using an alkaline absorbing solution as a SOx absorber.
As a process for treating metallic mercury in an exhaust gas, a removal process using an adsorbent such as active carbon or selenium filter is known. It requires, however, a special adsorption-removal unit and hence is not suited for the treatment of a large volume of an exhaust gas, for example, discharged from a power plant.
As a method for treating metallic mercury in a large amount of an exhaust gas, proposed is a process comprising, in a flue, carrying out gas atomization of a chlorinating agent in a step upstream of a high temperature denitration apparatus, oxidizing (chlorinating) mercury on a denitration catalyst to convert it into water-soluble mercury chloride, and absorbing it in a wet desulfurization apparatus disposed downstream of the denitration apparatus (refer to, for example, Japanese Patent Provisional Publication No. 230137/1998)).
As a unit and technology for gas atomization into a flue, an NH3 atomizer of a denitration apparatus has been put to practical use. A similar method can be employed for gas atomization of a chlorinating agent. Hydrogen chloride gas is however highly corrosive so that excessive addition of a chlorinating agent causes corrosion of a flue or downstream apparatuses in the system, resulting in the shortening of the plant life.
In order to prevent the influence of a chlorinating agent on the apparatus such as corrosive damage and to improve reliability, a system for measuring the mercury concentration of an exhaust gas after wet desulfurization by a mercury monitor and adjusting the feed amount of a chlorinating agent based on the mercury concentration after desulfurization has been proposed (refer to, for example, Japanese Patent Provisional Publication No. 2001-198434).
The feed amount of a chlorinating agent can be adjusted relatively easily when a high-purity hydrogen chloride gas is used directly as a chlorinating agent, but it is expensive and therefore is not economical for the treatment of a large amount of an exhaust gas.
In addition, there are no chlorinating agents that can be atomized directly at the operation temperature (350 to 420° C.) of a denitration apparatus. Even neutral salt slurry such as NaCl cannot be decomposed at the operation temperature of a denitration apparatus so that it is not only ineffective but also may presumably cause blockage.